In ion mobility spectrometry devices, separation of gas-phase ions is accomplished by exploiting variations in ion drift velocities under an applied electric field arising from differences in ion mobilities. One well-known type of ion mobility spectrometry device is the High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) cell, also known by the term Differential Ion Mobility Spectrometry (DIMS) cell, which separates ions on the basis of a difference in the mobility of an ion at high field strength (commonly denoted as Kh) relative to the mobility of the ion at low field strength (commonly denoted as K). Briefly described, a FAIMS cell consists of a pair of spaced apart electrodes that define therebetween a separation region through which a stream of ions is directed. An asymmetric waveform comprising a high voltage component and a lower voltage component of opposite polarity, together with a DC voltage (referred to as the compensation voltage, or CV) is applied to one of the electrodes. When the ion stream contains several species of ions, only one ion species is selectively transmitted through the FAIMS cell for a given combination of asymmetric waveform peak voltage (referred to as the dispersion voltage, or DV) and CV. The remaining species of ions drift toward one of the electrode surfaces and are neutralized. The FAIMS cell may be operated in single ion detection mode, wherein the DV and CV are maintained at constant values, or alternatively the applied CV may be scanned with time to sequentially transmit ion species having different mobilities.
FAIMS cells may be used for a variety of purposes, including providing separation or filtering of an ion stream prior to entry into a mass analyzer. An example of this type of application is disclosed in U.S. Pat. No. 6,822,224 to Guevremont. When a FAIMS apparatus is used in isolation, the identity of the peaks appearing in FAIMS CV spectra can not always be unambiguously confirmed due to the unpredictable changes in Kh at high electric fields. One way to extend the capability of instruments based on the FAIMS concept is to provide a way to determine the make-up of the FAIMS CV spectra more accurately, for example, by introducing ions from the FAIMS device into a mass spectrometer for mass-to-charge (m/z) analysis. Likewise, use of a FAIMS apparatus to separate ionic species prior to their introduction into a mass spectrometer can enhance the resulting mass spectrum by eliminating noise-producing background ions as well as by enabling the separate detection of some ionic species, such as isomers, that have identical mass-to-charge ratios.
Unfortunately, sequential use of a FAIMS spectrometer and a mass spectrometer in the fashion described above can lead to poor detected signal intensity, since the percentage transmission through the combined apparatus will be the product of the separate transmission percentages through each of the FAIMS and MS devices. Thus, in those cases in which a mass spectrometer alone may provide adequate analytical capabilities, it is often necessary to remove all or a portion of a front-end FAIMS apparatus to obtain the best results. Further, differing methods of interfacing to ion sources may necessitate replacement of one ion source for another at the same time that the FAIMS apparatus is removed. For instance, in some conventional FAIMS-MS hybrid systems, the FAIMS electrode set must be removed from the mass spectrometer and replaced with a standard ion source housing in order to pass all ions into the MS instrument. This takes a few minutes to accomplish and requires the operator to be present to manually remove the source.
Further, whereas mass spectrometers generally operate under high vacuum conditions, FAIMS analyzers operate at atmospheric or near-atmospheric pressure, since the differential mobility that is exploited is caused by interactions of ions with an inert molecular bath gas, typically containing helium. The gas or gases supplied to the FAIMS analyzer typically provide the additional functions of sweeping or carrying the ions through the apparatus and de-solvating charged liquid droplets. The carrier gas portion, after sweeping ions through the FAIMS apparatus from an ion inlet to an ion outlet, must be subsequently substantially vented to atmosphere so as to not interfere with the evacuated environment of the mass spectrometer. The de-solvating gas portion typically flows outward through the FAIMS ion inlet in counter flow to the movement of ions. Such configurations can lead to undesirably high consumption of helium or other inert gases. Also, the counter current gas flowing out of the entrance can lead to difficulties in coupling to certain ion sources, such as nano-electrospray sources. Further, operation with negative ion Atmospheric Pressure Chemical Ionization (APCI) sources is not possible due to arcing that occurs when too much helium is introduced into the FAIMS gas mixture. Still further, FAIMS operation at too high pressure (e.g., near-atmospheric pressure) can lead to too-long residence times within the FAIMS apparatus that interfere with mass spectrometer operation.
Accordingly, there are needs in the art for a FAIMS apparatus that does not require separation from a mass spectrometer in order to pass a substantial fraction of ions from an ion source into a mass spectrometer, for FAIMS apparatus having reduced gas consumption, easier coupling to nano-electroospray and APCI ion sources (especially APCI sources operated in negative ion mode), reduced ion residence time, and better integration with mass spectrometers. The present invention addresses such needs.